1. Field of the Invention
The present invention relates to composite or coated sulfonated poly(arylether) membranes for reverse osmosis ("RO"), ultrafiltration ("UF") and microfiltration ("MF") made by a novel process and their method of use.
2. Description of the Prior Art
The present commercially available membranes for water desalination by reverse osmosis are derived from two basic classes of polymers: cellulosic (predominantly cellulose acetates) and the newer generation condensation products consisting of polyamides, polyamidohydrazides, polyureas and polyetherureas. The cellulosic membranes are susceptible to microbiological attack, compaction at higher temperatures and pressures, and are limited to a relatively narrow feed pH range. These membranes do, however, possess fairly good resistance to low levels of chlorine, a popular disinfectant and cleaning chemical used in water desalination and other separation processes. The second group of polymers (which are suitable for ultrafiltration as well as RO) in general exhibit improved transport properties at given applied pressures and stability over a wider range of pH compared to the cellulosic membranes. Unfortunately, all of these membranes made from the newer generation of polymers suffer from poor resistance to continual exposure to oxidizing agents such as chlorine in a RO application. This "tradeoff" in sensitivities results in economically disadvantageous chlorine removal in many common feed streams or loss of membrane permselectivity due to oxidative degradation. Thus, this kind of sensitivity limits or even prevents their use in potable water applications and especially food and beverage, medical, biochemical, and pharmaceutical applications where chlorination and other similar oxidative cleaners or sterilants are commonly employed.
In recent years polysulfone type polymers have been extensively used in the manufacture of ultrafiltration membranes for use in many applications because of excellent hydrolytic stability and high temperature properties. The polysulfone polymers that are commercially available today, UDEL.RTM. polysulfone manufactued by Union Carbide Corp. and Victrex.RTM. polyethersulfone manufactured by ICI, also possess acceptable chlorine resistance when exposed periodically during cleaning. Therefore, these polysulfone type polymers are extensively used for membrane applications in the dairy and food processing areas that require daily sanitizations with chlorine and high temperature membrane cleaning regimens of 1% caustic and 1% phosphoric acid. Polysulfone type membranes have also found extensive use in pharmaceutical and biotechnology application areas and perform very well under most circumstances.
In recent years, sulfonated polyarylethers, and particularly sulfonated polyarylether sulfones have been examined for membrane separation applications due to their outstanding chemical and thermal stability. In addition to demonstrating good RO membrane flux and salt rejections at pressures greater than 300 psig, these membranes have demonstrated the ability to operate at continuous chlorine exposure levels and pH extremes that would destroy both of the previously mentioned classes of polymer membranes. Thus the chlorine tolerance and hydrolytic stability of the sulfonated polyarylethers would, at first blush, appear to make them particularly well suited for desalination of a wide range of aggressive feed streams represented by such conditions ranging from natural brackish waters, industrial effluents, sewage effluents, mining waters, agricultural run-off, etc. and many applications other than water desalination and water recovery mentioned previously.
The problem with these membranes has been their inability to achieve commercially attractive fluxes and salt rejections reproducibly at economical applied pressures. Acceptable performance for brackish water (low pressure RO) desalination would typically be: at least 15 GFD (gallons/ft.sup.2 -day) water flux and 95% or better salt rejection (5% or less salt passage) at 200 psig net driving pressure (NDP) or alternatively, an equitable tradeoff in these properties. Consistent with the principles of reverse osmosis, higher applied pressures will give higher flux and salt rejection, but at a penalty of added power cost for the extra pressure. It is therefore desirable to develop membranes which produce adequate flux and salt rejections at lower applied pressures.
The prior art involving sulfonated polyarylether membranes reveals that in order to obtain the desired transport properties on brackish (2000-5000 ppm NaCl) feeds of 15 or more GFD flux and 95% or better salt rejection, applied pressures of 300-600 psig or even more had to be used. The predominant membrane types developed were asymmetric, and consisted of thick (1-20 mil) anisotropic structures in which the permselectivity occurs at the thin dense film top side of this structure. The thin top film is integral with and supported by a progressively more porous understructure. Better results were found with thin film composite membranes in which selected laboratory samples developed by Graefe, et al (Office of Water Research and Technology, Report No. 2001-20) achieved low pressure goal performance at 250 psig (32 GFD flux and 94.5% salt rejection). This latter membrane example consisted of a thin film laminate prepared by brush coating a solution of sulfonated polysulfone onto a porous polysulfone substrate which was pretreated with aqueous lactic acid. The purpose for the latter solution was claimed to prevent intrusion of the sulfonated polysulfone solution into the pores of the porous substrate. This thin film composite membrane while demonstrating potential, suffered from an awkward porous substrate pretreatment step and lack of performance reproductibility and never reached commercialization.
One such thin film composite membrane which was carried further in its development was the hollow fiber composite system developed by Schiffer, et al. This membrane was prepared by coating an alcoholic solution of a highly sulfonated polysulfone (free acid form) onto a porous hollow-fiber polysulfone substrate. The thin film was claimed to have been crosslinked via the thermal treatment applied after solution deposition, though no proof of this was actually given. Performance was modest--at 6 GFD flux and up to 95% salt rejection on a 3500 ppm NaCl feed at 400 psig applied pressure. Problems with this membrane included inherent fragility of the coated fibers and ultimately inconsistent performance results on test in the full scale element mode.
The vast majority of prior art sulfonated polyarylether membranes has consisted of either (impractical) laboratory scale, thick dense films or asymmetric structures. These types of structures have been incapable of providing at least 15 GFD flux with 95% salt rejection at economical applied pressures (under 400 psig) as required for RO applications. As a result, some researchers investigated thin film composite membrane designs, since this approach if carried out optimally yields maximum fluxes in conjunction with good salt rejections. This is consistent not only with theory but also with other known membrane structures in operation today. The limitations with prior art thin film composites rested with the techniques of fabrication and the polymer choices used.